<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    
        <p id="p">hello world</p>
    
    <script>
        /* before--在兄弟元素之前添加一个元素 
           after--在兄弟元素之后添加一个元素 */
        var p=document.getElementById("p");
        p.before("前面")
        p.after("后面")
    </script>
</body>
</html>